Maybe I was exaggerating, I guess it's not completely insignificant and it will become much more so in a few years' time, but it was clearly of no interest to the miner of that six-transaction block.  Maybe that was a particularly unusual case though.  You're right that even empty blocks do improve security by building on top of existing blocks, but it would be even better if it also dealt with some of the backlog that must have built up during that period.

For pooled miners, which receive transactions pre-hashed from a remote server, there seem to be distinct disadvantages to frequently breaking off work and asking the pool server for an update, especially with all the DDOS attacks against pools recently.